1. Field of the Invention
This invention relates to a cassette for housing therein a sheet-shaped image recording medium, such as X-ray photographic film or a stimulable phosphor sheet. This invention also relates to a device for conveying a sheet-shaped image recording medium out of a cassette.
2. Description of the Prior Art
X-ray photographic film has heretofore been known as a recording medium, on which a radiation image of an object, such as a human body, is recorded. Also, as a recording medium used for the same purposes, a sheet provided with a layer of a stimulable phosphor (hereinafter referred to as a stimulable phosphor sheet) has heretofore been known. Such stimulable phosphor sheets are disclosed in, for example, U.S. Pat. No. 4,258,264 and Japanese Unexamined Patent Publication No. 56(1981)-11395. When certain kinds of phosphors are exposed to radiation such as X-rays, .alpha.-rays, .beta.-rays, .gamma.-rays, cathode rays or ultraviolet rays, they store part of the energy of the radiation. Then, when the phosphor which has been exposed to the radiation is exposed to stimulating rays, such as visible light, light is emitted by the phosphor in proportion to the amount of energy stored thereon during its exposure to the radiation. A phosphor exhibiting such properties is referred to as a stimulable phosphor.
A radiation image of an object is recorded on a stimulable phosphor sheet by, for example, exposing the stimulable phosphor sheet to radiation, which has passed through the object. The stimulable phosphor sheet, on which the radiation image has been stored, is then scanned two-dimensionally with stimulating rays, such as a laser beam, which cause it to emit light in proportion to the amount of energy stored during exposure to the radiation. The light emitted by the stimulable phosphor sheet, upon stimulation thereof, is photoelectrically detected. In this manner, an electric image signal representing the radiation image can be obtained.
Ordinarily, the stimulable phosphor sheet or the X-ray photographic film described above is housed in a light-tight cassette or a light-tight magazine. Each cassette serves to house a single sheet-shaped image recording medium, such as a stimulable phosphor sheet, therein. In general, the cassette is constituted of a box member provided with an opening, through which a recording medium is to be fed into and out of the box member, at a portion, and a cover member, which is mounted on the box member via a swing axis approximately parallel to the sheet-shaped image recording medium having been housed in the box member and which opens and closes the opening by swinging around the swing axis. Such that the cover member may not open unnecessarily, the cassette is provided with a cover locking mechanism constituted of an engaging means, which is mounted on the box member, and an engaged means, which is mounted on the cover member and engaged with the engaging means.
When the cassette is used during an operation for recording a radiation image on the stimulable phosphor sheet, or the like, the cassette is located at a position for exposure to, for example, radiation having passed through an object. The stimulable phosphor sheet, or the like, which has been housed in the cassette, is exposed to radiation, and a radiation image is thereby recorded thereon.
When a cassette is loaded to an radiation image read-out apparatus for reading out a radiation image from, for example, a stimulable phosphor sheet on which the radiation image has been stored, it is necessary to release the lock of the cover member and then to move the cover member to a cover open position such that the housed recording medium may be taken out of the cassette or such that a new recording medium may be housed in the cassette. For that the working efficiency be high, it is desired that the operation for releasing the lock of the cover member and the operation for opening the cover member can be carried out automatically. A cassette wherein these two operations can be carried out automatically is disclosed in, for example, Japanese Unexamined Patent Publication No. 57(1982)-178234.
In the disclosed cassette, a movable member, which is located on the side of a box member and moves in a direction approximately parallel to the sheet-shaped image recording medium by receiving force from the exterior, and a cover member are connected with each other by an arm. When the cover member has been closed, the arm is approximately parallel to the direction, along which the movable member moves. The arm is pushed by the movable member, which moves with external force, and raised to an oblique orientation, and the cover member is thereby swung to a cover open position.
However, with the disclosed cassette, the arm is located in the box member which is usually as thin as approximately 15 mm. Therefore, it is very difficult to design the arm such that the arm may rise obliquely when being pushed. Also, even if the arm were designed in such a manner, a very large force would be required to push the arm and raise it to an oblique orientation. As described above, though the conventional cassette will theoretically enable an automatic cover opening operation, an automatic cover opening operation cannot practically be accomplished.